Alan — you beat me to it! I bet you have a well-thumbed copy of Eats, Shoots & Leaves on your bookshelf.

You might find it worth replacing the at sign in your signature block with something more wordy, like "AT THE ISP" as automatic email address harvesters regularly source from discussion boards. Most of the resultant junk and offensive mail will be caught by your own or your ISP's spam trap, but some will always get through.

I'm more curious as to why such a small collection should have any duplicates at all.

Simply a bug -- I noticed this a while after the puzzles were used, so I thought it would be confusing to retrospectively fix it. Actually, I've managed to do the same puzzles more than once without noticing until completion, and without any lessening of enjoyment. It's taken you a systematic approach to spot it -- the 30 or 40 thousand people who've grabbed the puzzles to do 'em haven't noticed :)

Some of them could solve only the first one. some only the second.
This is why the germans invented the:
"Doppelt hält besser"
It applies not only for humans, but also for computer programs.
And this is not a "bug" but a "feature".

Thanks Sam. I actually wrote my duplicate checker for my own collection. My program creates new puzzles on the fly, but I wanted to ensure every one was unique. I tested the checker on your archive thinking it would pass, but was surprised when it didn't. Still, it at least proved the checker was working as I intended.

My spare machine's been working flat-out generating, saving and checking for duplicates for almost 2 hours now. So far there are none -- gotta be a good sign. Unfortunately it's already creating them far quicker than it can check them so I expect it will fall over sooner or later... Ho-hum